1ST MEIOTIC DIVISION ABNORMALITIES IN HUMAN OOCYTES - MECHANISM OF TRISOMY FORMATION

Trisomy is the single most frequent type of chromosome abnormality in humans and has considerable impact on many aspects of human pathology. It arises most commonly through ''nondisjunction'' at maternal meiosi s I, but the underlying mechanism of formation remains obscure. Analys is of 100 haploid oocytes at second meiotic metaphase shows that the o nly type of chromosome abnormality compatible with trisomy formation a fter fertilisation is the presence of single chromatids in addition to , or replacing, whole chromosomes. The mechanism resulting in the pres ence of single chromatids is considered to be precocious division of u nivalents or dyads at first meiotic anaphase.